Conventionally, a connector has been known which is electrically connected to a plurality of front-surface electrodes arranged side by side on a front surface of a planar sensor element and to a plurality of back-surface electrodes arranged side by side on a back surface of the sensor element. For example, Patent Document 1 describes a gas sensor that includes a planar sensor element configured to detect gas, conductors such as lead wires, and a connector configured to connect the sensor element and the conductors. The connector of the gas sensor includes contact fittings that are electrically connected to the sensor element, and housings that holds the contact fittings. The sensor element is clamped by two housings. FIG. 10 illustrates contact fittings 201a to 201c and a housing 200 of such related art. As illustrated, the contact fittings 201a to 201c are provided with protrusions 202a to 202c, respectively, at which the contact fittings 201a to 201c come into contact with and are electrically connected to front-surface electrodes and back-surface electrodes of the sensor element. The contact fittings 201a to 201c are also provided with hooks 203a to 203c, respectively, which are orthogonal to the length of the contact fittings 201a to 201c and bend outward from the contact fittings 201a to 201c. The contact fittings 201a to 201c are arranged in a direction orthogonal to the length of the contact fittings 201a to 201c and secured to the housing 200 by the hooks 203a to 203c.     Patent Document 1: U.S. Pat. No. 5,246,562 (FIG. 7 and FIG. 8)